Section II. GENERAL DISCUSSION

TB 9-4931-539-35

i. Time . The designation of an instant on a selected time scale, astronomical or

atomic. It is used in the sense of the time of day.

j. Time Interval. Indicates the duration of a segment of time without reference to

when the time interval begins and ends. Time interval may be given in seconds of time.

GENERAL DISCUSSION

4. VLF Tracking System. Since frequency is the reciprocal of time, the atomic second

is used as the standard for frequency. Radio station WWVB at Fort Collins, CO,

transmits a standard 60 kHz carrier frequency directly traceable to NIST. This signal

carries a basic accuracy of 2 parts in 1011 and is normally transmitted 24 hours a day

over the continental U.S. In addition to WWVB, there are a number of other stations

which transmit carrier frequencies with equal accuracy. These frequencies are listed in

tables 1 and 2 below. Occasionally, due to poor reception caused by weather, sunspots,

transmission path length, etc., the standard VLF signals cannot be suitably used. An

alternative is the TIMESHARED OMEGA system which transmits repetitively on

assigned time segments of a 10-second duration. Basic accuracy is to parts in 1011.

5. System Components. A local frequency standard can be maintained to within a few

parts in 1010 by phase comparison with a received VLF carrier transmission. This VLF

transmission is picked up by the Model 599-700R Ferrite Loop Antenna and applied to

the Model 599J/K VLF Tracking Receiver. Basic accuracy of the 599J/K is 2 parts in

1011. The VLF receiver and phase comparator provide a precision measurement of

relative phase between the transmitted VLF carrier frequency and the local time base

reference (Model 105A or FS-323). The output of the phase comparator is used to drive a

phase shifter for automatically locking the local time base reference with the selected

VLF carrier. The phase of the local time base frequency is thus locked to the phase of

the received VLF carrier signal. A strip chart recorder responds to error signals

indicating slight phase deviation between the two signals and provides a visual

indication to allow the operator to adjust the local standard. When the frequency of the

local oscillator has been calibrated, it becomes the local standard for calibration of

counters, oscilloscopes, etc. A frequency difference meter (Model 527E or FDM-2100) is

connected between the local oscillator and the instrument under test and is used to

determine the fractional frequency difference between the two. This provides a visual

means for adjusting the TI to that of the standard.